Stacked semiconductor chip RGBZ sensor

ABSTRACT

An apparatus is described that includes a first semiconductor chip having a first pixel array. The first pixel array has visible light sensitive pixels. The apparatus includes a second semiconductor chip having a second pixel array. The first semiconductor chip is stacked on the second semiconductor chip such that the second pixel array resides beneath the first pixel array. The second pixel array has IR light sensitive pixels for time-of-flight based depth detection.

RELATED CASES

This application is a continuation of and claims the benefit of U.S.patent application Ser. No. 15/693,544, titled “STACKED SEMICONDUCTORCHIP RGBZ SENSOR”, filed Sep. 1, 2017, which is a continuation of andclaims the benefit of U.S. patent application Ser. No. 15/360,906,titled “STACKED SEMICONDUCTOR CHIP RGBZ SENSOR”, filed Nov. 23, 2016,which is a continuation of and claims the benefit of U.S. patentapplication Ser. No. 14/579,882, titled “STACKED SEMICONDUCTOR CHIP RGBZSENSOR”, filed Dec. 22, 2014, which is incorporated by reference in itsentirety.

FIELD OF INVENTION

The field of invention pertains generally to the electronic arts, and,more specifically, to a stacked semiconductor chip RGBZ sensor.

BACKGROUND

Many existing computing systems include one or more traditional imagecapturing cameras as an integrated peripheral device. A current trend isto enhance computing system imaging capability by integrating depthcapturing into its imaging components. Depth capturing may be used, forexample, to perform various intelligent object recognition functionssuch as facial recognition (e.g., for secure system un-lock) or handgesture recognition (e.g., for touchless user interface functions).

One depth information capturing approach, referred to as“time-of-flight” imaging, emits light from a system onto an object andmeasures, for each of multiple pixels of an image sensor, the timebetween the emission of the light and the reception of its reflectedimage upon the sensor. The image produced by the time of flight pixelscorresponds to a three-dimensional profile of the object ascharacterized by a unique depth measurement (z) at each of the different(x,y) pixel locations.

As many computing systems with imaging capability are mobile in nature(e.g., laptop computers, tablet computers, smartphones, etc.), theintegration of time-of-flight operation along with traditional imagecapture presents a number of design challenges such as cost challengesand packaging challenges.

SUMMARY

An apparatus is described that includes a first semiconductor chiphaving a first pixel array. The first pixel array has visible lightsensitive pixels. The apparatus includes a second semiconductor chiphaving a second pixel array. The first semiconductor chip is stacked onthe second semiconductor chip such that the second pixel array residesbeneath the first pixel array. The second pixel array has IR lightsensitive pixels for time-of-flight based depth detection.

An apparatus is described that includes means for receiving visiblelight and IR light at a surface of a first pixel array. The apparatusalso includes means for sensing the visible light but not the IR lightwith the first pixel array, where, the IR light passes through the firstpixel array. The apparatus also includes sensing the IR light with asecond pixel array located beneath the first pixel array.

FIGURES

The following description and accompanying drawings are used toillustrate embodiments of the invention. In the drawings:

FIGS. 1a and 1b show an embodiment of a stacked semiconductor chip RGBZsensor;

FIG. 2 shows a depiction of the operation of a stacked semiconductorchip RGBZ sensor;

FIGS. 3a through 3i show a method of manufacturing a stackedsemiconductor chip RGBZ sensor;

FIGS. 4a through 4d show different image sensor circuitry distributions;

FIG. 5 shows a method performed by a stacked semiconductor chip RGBZsensor;

FIG. 6 shows an embodiment of a 2D/3D camera system.

FIG. 7 shows an embodiment of a computing system having a 2D/3D camerasystem.

DETAILED DESCRIPTION

An “RGBZ” image sensor is an appealing solution for achieving bothtraditional image capture and time of flight depth profiling from withina same camera package. An RGBZ image sensor is an image sensor thatincludes different kinds of pixels, some of which are sensitive tovisible light (the RGB pixels) and others of which are used to measuredepth information (the time-of-flight or “Z” pixels).

In a common implementation, time of flight pixels are designed to besensitive to IR light because, as mentioned above, IR light is used forthe time-of-flight measurement so that the time-of-flight measurementdoes not interfere with the traditional imaging functions of the RGBpixels. The time-of-flight pixels additionally have special associatedclocking and/or timing circuitry to measure the time at which light hasbeen received at the pixel. Because the time-of-flight pixels aresensitive to IR light, however, they may also be conceivably used (e.g.,in a second mode) as just IR pixels and not time-of-flight pixels (i.e.,IR information is captured but a time of flight measurement is notmade).

Integrating both RGB pixels and Z pixels into a same package shouldreduce both size and cost as compared to solutions in which the RGBpixels and Z pixels are contained in separate packages. FIGS. 1a and 1bshow a “stacked” RGBZ image sensor 100 in which a first type of pixelarray 101 (e.g., an RGB pixel array) is stacked on top of a second typeof pixel array 102 (e.g., an IR or Z pixel array). Here, the stackedstructure 100 includes functional pixels of a first type 107 (RGB), asopposed to mere filters, stacked on a lower layer of functional pixelsof a second type 108 (Z). As will be discussed in more detail furtherbelow, in operation, the upper pixel array 101 is implemented as abackside illumination pixel array and the lower pixel array 102 isimplemented as a frontside illumination pixel array.

As observed in the embodiment of FIGS. 1a and 1b , backside interconnectmetallization layering 109, an RGB filtering layer 110 and a micro-lensarray 112 reside on the backside of the semiconductor die 104 of theupper pixel array 101. The backside metallization layering 109 helpssupport chip-to-chip vias 116 between the pair of pixel arrays 101, 102.The RGB filtering layer 110 includes different colored filters 111_1,111_2 to define the RGB pixels of the upper pixel array 101. The microlens array 112 is formed on an upper surface of the overall structure100 along with wire bond pads 118. Wire bonds formed on the wire bondpads 116 may land on a package substrate. In some embodiments lowersubstrate 114 corresponds to the package substrate while in otherembodiments lower substrate 114 corresponds to another semiconductorchip which is mounted on the package substrate (in the later embodimentsthe package substrate is not shown in FIG. 1b ).

The upper pixel array 101 also includes frontside metallization layering103 to form the transistor components (such as gate and source/drainelectrodes) and corresponding layer(s) of interconnect that togetherform the circuitry that is integrated on the upper pixel array 101.Notably, the upper pixel array 101 includes through substrate vias 115that extend through the semiconductor substrate 104 of the upper pixelarray 101 to connect nodes of the metallization layering 103 to nodes ofmetallization layer 109.

The lower pixel array 102 includes frontside interconnect metallizationlayering 106 that faces the frontside interconnect metallization 103 ofthe upper pixel array 101. Frontside metallization layering 106 is usedto form the transistor components (such as gate and source/drainelectrodes) and corresponding layer(s) of interconnect that togetherform the circuitry that is integrated on the lower pixel array 102. Thefrontside metallization layer 106 of the lower pixel array 102 alsoinclude nodes that are connected to vias 116 that connect to nodes ofthe backside metallization layering 109 of the upper pixel array 101.These vias 116, like vias 115, are also implemented asthrough-substrate-vias of the upper pixel array 101.

The frontside metallization layer 106 of the lower pixel array 102further includes additional nodes that are connected to vias 117 thatconnect to bond pads 118. Like vias 115 and 116, vias 117 are alsoimplemented as through-substrate-vias of the upper pixel array 101.Although FIG. 1b indicates that all input/output signaling to/from upperpixel array 101 must pass through the frontside metallization layering106 of the lower pixel array 102, in various embodiments backsidemetallization layering 109 of the upper pixel array 101 may support viasthat directly connect to bond pads 118 thereby permitting signals topass between a bond pad and the upper pixel array 101 without having totransvers through the metallization layering 106 of the lower pixelarray 102.

Note that FIG. 1b shows an embodiment of a stacked RGBZ image sensoralong a particular slice that shows a first horizontal axis of the RGBpixel array. If, for example, a depiction of the same structure wereshown along a next horizontal axis that is closer to or farther awayfrom the GBGB . . . axis of FIG. 1b , the visible pixels of the upperpixel array 101 would instead demonstrate, e.g., a GRGR . . . patterninstead of an GBGB . . . pattern. Note also that in the embodiment ofFIG. 1b the Z pixels 108 are larger than the RGB pixels because, e.g.,silicon has a reduced sensitivity of IR light as compared to visiblelight and/or the Z pixels require larger well capacitance. The frontsidemetallization layers 103, 106 of the upper and lower pixel arrays 101,102 may also include respective light guide structures 113, 114 tocouple incident light that flows through the RGB pixels of the upperpixel array 102 into the Z pixels of the lower pixel array 102.

FIG. 2 depicts a method of operation of the stacked pixel arraystructure of FIGS. 1a and 1b . As observed in FIG. 2, light consistingof visible light and IR light 210 is incident upon the (e.g., backside)surface of the upper RGB pixel array 201. As is known in the art, theRGB filter layer(s) 110 formed on the upper pixel array 201 containsindividual, tile-like green 107_1, and blue 107_2 filters typicallyarranged in a pattern such as a Bayer pattern. Each individual RGBfilter passes not only visible light of its own specific color (e.g.,red in the case of an R pixel, blue in the case of a B pixel, etc.) butalso IR light. The specific color of the visible light and the IR lightimpinge upon the (e.g., backside) of the semiconductor substrate 104 ofthe upper pixel array 201. The pixels 107_1, 107_2 of the upper pixelarray 201 absorb the visible light and generate a respective detectionsignal for their respective colors in response thereto.

Unlike the visible light which is absorbed by the upper pixel array 201,by contrast, the IR light passes through the upper pixel array 201.Here, as a general matter of physics, visible light tends to be muchmore strongly absorbed by a semiconductor than IR light. As such, bymaking the semiconductor die 104 of the upper pixel array 201 layer thinenough, the upper pixel array 201 can be made to absorb and react to theincident visible light yet, at the same time, substantially pass theincident IR light.

Ideally therefore, the IR light 211 that was incident upon the upperpixel array 201 largely passes through the upper pixel array 201 to thelower pixel array 202. In an embodiment, the semiconductor die 105 ofthe lower pixel array 202 is made thicker than the semiconductor die 104of the upper pixel array 201 so that, unlike the upper pixel array 201,it absorbs the incident IR 211 light rather than passes it. As such, thepixels of the lower pixel array 202 absorb the IR light and generate arespective detection signal in response thereto. By absorbing the IRlight, any circuitry beneath the lower pixel array 202 (such as if lowersubstrate 114 is implemented as a semiconductor chip) is not disturbed.

In various implementations the Z pixels of the lower pixel array 202effectively perform “3D” time-of-flight depth measurements by generatingcharge as a function of the time between when an IR illuminator's lightwas flashed “on” and when the flash's reflected light is received at thelower pixel array 202. The Z pixels typically receive clock signals thateach have a known phase relationship with the illuminator's clocksignal. In one embodiment, there are four such clock signals (e.g., 0°,90°, 180° and 270° quadrature arms) provided to each Z pixel of thelower pixel array.

Here, clocking a Z pixel with clocks of differing phase (e.g., through 4sequential exposure times) will collect different amounts of charge fora same light flash. Collected charge signals from the different clockscan be combined to generate a specific time-of-flight value for theregion where the nodes reside. In one implementation, such combinationis made by an image signal processor (e.g., integrated on a host systemsuch as a processor or applications processor semiconductor chip).

Note that the stacked pixel array structure of FIG. 2 naturally lendsitself to simultaneously being able to detect visible images with theupper RGB pixel array 201 while detecting IR light for time of flightmeasurements with the lower Z pixel array 202.

FIGS. 3a through 3i show an exemplary process for manufacturing thestacked pixel array structure. As observed in FIG. 3a , the upper pixelarray 301 is formed (e.g., according to a first wafer manufacturingprocess) with frontside metallization 303 that includes correspondinglight guides 114 each formed over groups of visible light pixel regionsthat are commensurate in size with the Z pixels associated with thelower pixel array (e.g., a 4×4 square of RGB pixels containing a pair ofR pixels, a G pixel and a B pixel).

As is known in the art frontside metallization 303 includes layers ofconductive traces 303 with corresponding layers of dielectric in betweenformed above the semiconductor substrate 304. The electronicinterconnect features typically include, for each pixel, one or morecontacts to the underlying silicon (e.g., to bias the pixel and/orpick-up the pixel's optically induced electrical signal) and wiringto/from other circuits located around the periphery of the active pixelarray that support the operation of the pixel array (e.g., senseamplifiers, row decoders, etc.). Transistor electrodes to implementthese circuits are usually formed at the lowest metallization layer. Forease of drawing the transistor electrodes are not depicted.

Care is taken in the layout of the metallization layering 303 to run theconductive traces as much as practicable along the edges of the pixelboundaries rather than above the pixels themselves (so that the tracesdo not block the incident light). One or more of the metallizationlayers may be used to form a shielding layer above the circuitry thatblocks incident light from reaching and disturbing the operation of theunderlying transistors.

In an embodiment, light guides 314 are formed by etching through stackeddielectric layers residing over the corresponding groups of RGB pixelswhere individual light guides are to be placed. The stacked dielectriclayers may correspond to the normal dielectric layers of layering 303without any metal traces. After etching appropriately located trenches,the trenches are filled (e.g., by deposition) with a high-index ofrefraction material that is transparent to IR light. The filling of thelight guide region with high-index material essentially forces internalreflection of IR light within the light guide and prevents crosstalktransmission of internal light to neighboring lightguides and/orneighboring pixels. Other approaches for forming the light guidesinclude etching an “annulus” or similar structure around the peripheryof the light guide feature boundary and filling the etching region withmetal (to again force internal reflection within the light guide) orleaving the etched region as an air gap.

Referring to FIG. 3b , the lower pixel array 302 is also formed (e.g.,according to a second wafer manufacturing process). The frontsidemetallization 306 is formed similarly to the frontside metallization 303of the upper pixel array including the formation of individual lightguides 313 above each of the Z pixel regions. The frontsidemetallization 306 also includes additional peripheral features 319 tosupport via connections to the upper pixel array or the bond pads. Asobserved in FIG. 3b note that the upper pixel array 301 is flipped withrespect to its depiction in FIG. 3a to prepare the upper pixel array 301for placement on the lower pixel array 302.

As observed in FIG. 3c , the inverted upper pixel array 301 is loweredonto the lower pixel array 302 according to a wafer-on-wafersemiconductor chip attach process or a die-on-die semiconductor chipattach process.

As observed in FIG. 3d , a lower dielectric layer 321 of backsidemetallization layering 109 is deposited over the backside of the upperpixel array 301. The dielectric layer 321 is then etched to form throughsubstrate vias 322 through the semiconductor substrate 304 of the upperpixel array. The through substrate vias 322 have different depths asobserved in FIG. 3d . A first depth extends only into the front sidemetallization layering 303 of the upper pixel array. A second depthextends deeper into the front side metallization layering 306 of thelower pixel array.

In an embodiment the two different etch depths are controlled by maskingone another out. That is, while one of the etch depths is being etched,the regions where the other etch depth is to be etched are covered by amasking layer. As such, two different masks sequences are used to formthe two etch depths. This permits different etch processes (e.g. alonger etch time for the deeper etch). The etching may be perfomed,e.g., with a reactive ion etch (RIE) or deep reactive ion etch (DRIE).

As observed in FIG. 3e , the backside metallization layering 309 of theupper pixel array 301 is completed including the filling of the etchedregions with conductive material (e.g., metal) and the formation offeatures 323 that electrically connect the upper and lower pixel arrays301, 302 to one another. In one approach, the filling of the etchedregions with conducting material first includes filling the etchedregions with insulating material (e.g., silicon dioxide) to form aninsulating liner within the trenches and then etching the insulatingmaterial similar to the manner in which the regions were initiallyetched in order to form openings. Again these etches may be performedwith different respective masks. After the regions are re-etched theyare filled with metal.

FIG. 3f shows the formation of a filtering layer 310 over the backsidemetallization layering 310. The filtering layer may be formed by forminga mordent or transparent layer over the surface of the backsidemetallization layering 310. Then, visible light filters of a first type311_1 (e.g., a green colored “G” filter) are formed in the mordent ortransparent layer by dying the layer the appropriate color (G) in theappropriate region. The dying may be implemented by heat transferring amordent dye through a photoresist mask into a mordent layer and thenstripping the mask, or, imbibing a dye into a transparent layer througha photoresist mask and then stripping the mask. Here, the photoresistand masking is patterned so as to expose the regions of interest (the Gpixel regions) and block out the other others regions (the R and Bregions). The process then repeats for the other regions and colors asappropriate (e.g., R region 311_2 and B region (not shown in FIG. 3f )).

As observed in FIG. 3g the filtering layer 310 is etched to form etchedregions 324. The etched regions 324 may all have a same depth or havedifferent depths depending on implementation. Again, individual maskingcan be used to form different etch depths. As observed in FIG. 3g , inan embodiment, at least some of the etch depths extend through the upperpixel array semiconductor substrate 304 and into the front sidemetallization 306 of the lower pixel array 302. These etched regions, aswill become more apparent below, support through substrate vias thatconnect to I/O bond pads of the stacked RGBZ pixel array. Other etchdepths (not shown) may extend only into the backside metallizationlayering 309 of the upper pixel array 301 so that I/O bond pads may bedirectly connected to features within this layer.

FIG. 3h shows the structure after the through substrate vias are formedin the etched regions 324 and bond pads 325 are placed thereon. The viasmay be formed similar to the vias of FIG. 3 e.

FIG. 3i shows the completed stacked RGBZ pixel array after the formationof the micro lens array 312 over the filtering layer 310. Themicro-lenses can be formed by any of a number of various processes suchas: 1) coating and baking one or more photoresist layers on theunderlying structure, patterning the photoresist layers into, e.g.,circles/cylinders representing the micro-lens array and then melting thephotoresist circles/cylinders into the shape of the micro-lenses; 2)performing the process of 1) above on a layer on a transparent layer(e.g., fused silica) and using the melted photoresist as a mask for aRIE etch into the transparent layer (which completes the form of fullermicro-lenses into the transparent layer); 3) micro-jetting dropletsaimed on the underlying structure in the array pattern and solidifyingthe droplets.

FIGS. 4a through 4d show different possible architectures for thestacked pixel array structure discussed above. Understanding that aspectrum of different architectural possibilities exist, FIG. 4a showsan embodiment toward one end of the spectrum while FIG. 4b shows anembodiment toward another end of the spectrum. The spectrum ends areclassified as a function of how much image sensor functionality otherthan a pixel array is integrated on the stacked semiconductor diestructure.

Recalling that transistors may be formed, e.g., on the outer peripheryof both pixel arrays, such transistors are generally used to formcircuits that perform image sensor functionality. To the extent moreimage sensor functionality is integrated on one of the dies having anactive pixel array, the numbers of such transistors (and thecorresponding semiconductor surface area consumed by them) willincrease, and, to the extent less image sensor functionality isintegrated on a die having an active pixel array the numbers of suchtransistors (and the corresponding semiconductor surface area consumedby them) will decrease.

As is understood in the art, image sensors can generally be viewed asincluding not only an active pixel array but also pixel array circuitry,analog-to-digital conversion circuitry and timing and control circuitry.The pixel array circuitry generally includes circuitry that directlycommunicates to/from the active pixel array itself. Examples includesense amplifiers, row and column address decoders and, in the case of Zpixels for time-of-flight measurements, at least one clock signal per Zpixel for performing the time-of-flight measurement. Analog-to-digitalcircuitry is responsible for converting the analog signals detected fromthe incident optical signals to digital values. The timing and controlcircuitry is responsible for providing the appropriate clocking signalsand control signals to the pixel array circuitry and theanalog-to-digital converter circuitry.

Toward one end of the spectrum, observed in FIG. 4a , little suchfunctionality is placed on either pixel array die, leaving the bulk ofsuch functionality to be placed on the third, lower substrate which isimplemented as a semiconductor chip. Toward the other end of thespectrum, observed in FIG. 4b , large amounts of such functionality areplaced on both pixel array dies. In the particular embodiment of FIG. 4b, the upper die corresponds to an entire RGB image sensor and the lowerdie corresponds to an entire time-of-flight image sensor. In this case,the lower substrate can be implemented as a package substrate.

A large range of architectural possibilities exist between these twospectral end regions. For example, as observed in FIG. 4c , only thepixel array circuitry is implemented on their respective pixel arraydies leaving the timing and control circuitry and analog-to-digitalcircuitry for both image sensors to be implemented on the lowestsemiconductor chip die.

As another example, observed in FIG. 4d , the lower pixel array dieincludes the timing and control circuitry and ADC circuitry for both theupper and the lower pixel arrays. Note that in the case where the lowerpixel array die contains circuitry that supports the upper pixel array,the top surface of the lower pixel array should have pads/lands thatconnect to circuitry internal to the lower pixel array die rather than athrough-substrate-via of the lower pixel array die. From FIGS. 4athrough 4d , one of ordinary skill will recognize that a wide range ofarchitectural possibilities exist where the various circuits for thepair of image sensors, or portions thereof, may potentially reside onany of the upper pixel array die, the lower pixel array die and thelower substrate semiconductor chip (if any).

FIG. 5 shows a method performed by the RGBZ image sensor embodimentsdiscussed above. As observed in FIG. 5 incident visible light andincident IR light are received at a surface of a first pixel array 501.The visible light but not the IR light is sensed with the first pixeland the IR light passes through the first pixel array 502. The IR lightis sensed with a second pixel array located beneath the first pixelarray 503.

FIG. 6 shows an integrated traditional camera and time-of-flight imagingsystem 600. The system 600 has a connector 601 for making electricalcontact, e.g., with a larger system/mother board, such as thesystem/mother board of a laptop computer, tablet computer or smartphone.Depending on layout and implementation, the connector 601 may connect toa flex cable that, e.g., makes actual connection to the system/motherboard, or, the connector 601 may make contact to the system/mother boarddirectly.

The connector 601 is affixed to a planar board 602 that may beimplemented as a multi-layered structure of alternating conductive andinsulating layers where the conductive layers are patterned to formelectronic traces that support the internal electrical connections ofthe system 600. Through the connector 601 commands are received from thelarger host system such as configuration commands that write/readconfiguration information to/from configuration registers within thecamera system 600.

An “RGBZ” image sensor composed of an RGB pixel array implemented on afirst semiconductor die that is stacked on a second semiconductor diehaving IR pixels to, e.g., implement time-of-flight pixels are within asemiconductor chip package 603 that is mounted to the planar board 602.The stacked RGBZ image sensor includes an upper RGB pixel array havingdifferent kinds of pixels that are sensitive to visible light(specifically, a subset of R pixels that are sensitive to visible redlight, a subset of G pixels that are sensitive to visible green lightand a subset of B pixels that are sensitive to blue light). A lower Zpixel array has pixels that are sensitive to IR light. The RGB pixelsare used to support traditional “2D” visible image capture (traditionalpicture taking) functions. The IR sensitive pixels are used to support3D depth profile imaging using time-of-flight techniques. Although abasic embodiment includes RGB pixels for the visible image capture,other embodiments may use different colored pixel schemes (e.g., Cyan,Magenta and Yellow). The RGBZ image sensor may also include ADCcircuitry and timing and control circuitry for both pixel arrays.

The planar board 602 may likewise include signal traces to carry digitalinformation provided by the ADC circuitry to the connector 601 forprocessing by a higher end component of the computing system, such as animage signal processing pipeline (e.g., that is integrated on anapplications processor). Note that in other embodiments an image signalprocessing pipeline or at least some form of digital signal processingperformed on the ADC output pixel stream may be performed with digitallogic circuitry on a semiconductor chip that is integrated into thecamera system 600.

A camera lens module 604 is integrated above the pixel arrays of theRGBZ image sensor 603. The camera module 604 contains a system of one ormore lenses to focus light received through an aperture of the RGBZimage sensor package 603.

An illuminator 607 composed of a light source array beneath an aperture606 is also mounted on the planar board 602. The light source array maybe implemented as an array of vertical cavity side emitting lasers(VCSELs) or light emitting diodes (LEDs) implemented on a semiconductorchip that is mounted to the planar board 601. Alternatively, a singlelight source may be used (e.g. a single VCSEL or LED as opposed to anarray). A light source driver is coupled to the light source array tocause it to emit light with a particular intensity and modulatedwaveform.

In an embodiment, the integrated system 600 of FIG. 6 supports threemodes of operation: 1) 2D mode; 3) 3D mode; and, 3) 2D/3D mode. In thecase of 2D mode, the system behaves as a traditional camera. As such,illuminator 607 is disabled and the image sensor is used to receivevisible images through its RGB pixels. In the case of 3D mode, thesystem is capturing time-of-flight depth information of an object in thefield of view of the illuminator 607 and the camera lens module 604. Assuch, the illuminator is enabled and emitting IR light (e.g., in anon-off-on-off . . . sequence) onto the object. The IR light is reflectedfrom the object, received through the camera lens module 604 and sensedby the image sensor's time-of-flight pixels. In the case of 2D/3D mode,both the 2D and 3D modes described above are concurrently active.

FIG. 7 shows a depiction of an exemplary computing system 700 such as apersonal computing system (e.g., desktop or laptop) or a mobile orhandheld computing system such as a tablet device or smartphone. Asobserved in FIG. 7, the basic computing system may include a centralprocessing unit 701 (which may include, e.g., a plurality of generalpurpose processing cores) and a main memory controller 717 disposed onan applications processor or multi-core processor 750, system memory702, a display 703 (e.g., touchscreen, flat-panel), a local wiredpoint-to-point link (e.g., USB) interface 704, various network I/Ofunctions 705 (such as an Ethernet interface and/or cellular modemsubsystem), a wireless local area network (e.g., WiFi) interface 706, awireless point-to-point link (e.g., Bluetooth) interface 707 and aGlobal Positioning System interface 708, various sensors 709_1 through709_N, one or more cameras 710, a battery 711, a power managementcontrol unit 712, a speaker and microphone 713 and an audiocoder/decoder 714.

An applications processor or multi-core processor 750 may include one ormore general purpose processing cores 715 within its CPU 401, one ormore graphical processing units 716, a main memory controller 717, anI/O control function 718 and one or more image signal processorpipelines 719. The general purpose processing cores 715 typicallyexecute the operating system and application software of the computingsystem. The graphics processing units 716 typically execute graphicsintensive functions to, e.g., generate graphics information that ispresented on the display 703. The memory control function 717 interfaceswith the system memory 702. The image signal processing pipelines 719receive image information from the camera and process the raw imageinformation for downstream uses. The power management control unit 712generally controls the power consumption of the system 700.

Each of the touchscreen display 703, the communication interfaces704-707, the GPS interface 708, the sensors 709, the camera 710, and thespeaker/microphone codec 713, 714 all can be viewed as various forms ofI/O (input and/or output) relative to the overall computing systemincluding, where appropriate, an integrated peripheral device as well(e.g., the one or more cameras 710). Depending on implementation,various ones of these I/O components may be integrated on theapplications processor/multi-core processor 750 or may be located offthe die or outside the package of the applications processor/multi-coreprocessor 750.

In an embodiment one or more cameras 710 includes an integratedtraditional visible image capture and time-of-flight depth measurementsystem such as the system 600 described above with respect to FIG. 6.Application software, operating system software, device driver softwareand/or firmware executing on a general purpose CPU core (or otherfunctional block having an instruction execution pipeline to executeprogram code) of an applications processor or other processor may directcommands to and receive image data from the camera system. In the caseof commands, the commands may include entrance into or exit from any ofthe 2D, 3D or 2D/3D system states discussed above.

Embodiments of the invention may include various processes as set forthabove. The processes may be embodied in machine-executable instructions.The instructions can be used to cause a general-purpose orspecial-purpose processor to perform certain processes. Alternatively,these processes may be performed by specific hardware components thatcontain hardwired logic for performing the processes, or by anycombination of programmed computer components and custom hardwarecomponents.

Elements of the present invention may also be provided as amachine-readable medium for storing the machine-executable instructions.The machine-readable medium may include, but is not limited to, floppydiskettes, optical disks, CD-ROMs, and magneto-optical disks, FLASHmemory, ROMs, RAMs, EPROMs, EEPROMs, magnetic or optical cards,propagation media or other type of media/machine-readable mediumsuitable for storing electronic instructions. For example, the presentinvention may be downloaded as a computer program which may betransferred from a remote computer (e.g., a server) to a requestingcomputer (e.g., a client) by way of data signals embodied in a carrierwave or other propagation medium via a communication link (e.g., a modemor network connection).

In the foregoing specification, the invention has been described withreference to specific exemplary embodiments thereof. It will, however,be evident that various modifications and changes may be made theretowithout departing from the broader spirit and scope of the invention asset forth in the appended claims. The specification and drawings are,accordingly, to be regarded in an illustrative rather than a restrictivesense.

The invention claimed is:
 1. A method comprising: forming a backsideillumination pixel array including (i) pixels of a first type, and, (ii)for each pixel, a light guide structure between the pixel and a frontside surface of the backside illumination pixel array; forming a frontside illumination pixel array including (i) pixels of a second type,and, (ii) for each pixel, a light guide structure between the pixel anda front side surface of the front side pixel array; and stacking thebackside illumination pixel array on the front side illumination pixelarray.
 2. The method of claim 1, wherein: the pixels of the first typecomprise visible light sensitive pixels, and the pixels of the secondtype comprise infrared light sensitive pixels.
 3. The method of claim 1,wherein the pixels of the second type are larger than the pixels of thefirst type.
 4. The method of claim 1, comprising coupling correspondinglight guide structures of the backside illumination pixel array and thefront side illumination pixel array to allow incident light that passesthrough the pixels of the first type in the backside illumination pixelarray to flow to the pixels of the second type in the front side pixelarray.
 5. The method of claim 1, wherein the pixels of the first typeabsorb light of the first type and allow light of the second type topass through.
 6. The method of claim 1, wherein the front sideillumination pixel array is formed to be thicker than the backsideillumination pixel array.
 7. The method of claim 1, wherein forming thebackside illumination pixel array comprises: forming a semiconductor dielayer; and forming a front side metallization layer.
 8. The method ofclaim 1, wherein forming the front side illumination pixel arraycomprises: forming a semiconductor die layer; and forming aninterconnect metallization layer.
 9. The method of claim 1, whereinforming the backside illumination pixel array comprises: depositingconductive traces in an area around the pixels rather than over thepixels themselves.
 10. The method of claim 1, wherein forming thebackside illumination pixel array comprises etching the light guidestructure through stacked dielectric layers dispose over the pixels toform trenches.
 11. The method of claim 10, wherein forming the backsideillumination pixel array comprises filling the trenches with a materialthat includes a high index of refraction and that is transparent toinfrared light.
 12. The method of claim 1, wherein forming the backsideillumination pixel array comprises etching an annulus structure around aperiphery of the light guide structure to form an etched region or airgap.
 13. The method of claim 12, wherein forming the backsideillumination pixel array comprises filling the etched region with metal.14. The method of claim 1, wherein stacking the backside illuminationpixel array on the front side illumination pixel array comprisesaffixing the backside illumination pixel array to the front sideillumination pixel array using a wafer-on-wafer semiconductor chipattach process.
 15. The method of claim 1, wherein stacking the backsideillumination pixel array on the front side illumination pixel arraycomprises affixing the backside illumination pixel array to the frontside illumination pixel array using a die-on-die semiconductor chipattach process.
 16. The method of claim 1, comprising, after thebackside illumination pixel array is stacked on the front sideillumination pixel array, etching the stacked pixel arrays at differentdepths, to form etched regions.
 17. The method of claim 16, wherein thestacked pixel arrays are etched using a reactive iron etch.
 18. Themethod of claim 16, wherein the stacked pixel arrays are etched using adeep reactive iron etch.
 19. The method of claim 16, comprising fillingthe etched regions with a conductive materials.
 20. The method of claim1, comprising, after the backside illumination pixel array is stacked onthe front side illumination pixel array, forming a filtering layer overthe backside illumination pixel array.